Combi with water storage - efficiency advice required

MidlandsMJ

Hi,

I recently had a new boiler fitted which features a storage tank and combi backup (Glow Worm Ultrapower 170 SXI).

It claims to be the most efficient on the market ('SEDBUK A rated with NOx Class 5 rating - the best possible') and has a 170 litre storage tank. My question is regarding the most efficient way to utilise the hot water feature.

I live alone in a large house (3 bedrooms, 3 bathrooms inc. Shower + Bath), I use very little water daily, which of the following three options is the most cost effective for me:

A) Constantly have the hot water tank heated to a medium heat, if I ever do use the 170 litre capacity (Long shower?) the combi backup will kick in.

Heat the hot water tank every 2/3 days to a high temp and make use of the heated 170 litres - heating it again before it goes cold

C) Never heat the hot water tank and purely use the combi feature

Any help would be greatly appreciated, thanks.

macman

C) is cheapest.

HappyMJ

macman wrote: »

C) is cheapest.

If C were possible.

The boiler is not a combi. It's a system boiler you must use the built in cylinder to store hot water and draw from that when you want hot water.

It heats 170 litres of water to a preset temperature from 6am to midnight then every Wednesday at 2am it brings the water temperature up to 70 degrees to kill off any bacteria.

If you don't like that feature then get a cheaper boiler.

MidlandsMJ

Thanks guys, this demonstrates my ignorance.

HappyMJ - So am I right in thinking the 'combi backup' only comes into play if I have used all the water in the tank?

From what you are saying it sounds like turning off the hot water at certain intervals would not be worthwhile.

HappyMJ

MidlandsMJ wrote: »

Thanks guys, this demonstrates my ignorance.

HappyMJ - So am I right in thinking the 'combi backup' only comes into play if I have used all the water in the tank?

From what you are saying it sounds like turning off the hot water at certain intervals would not be worthwhile.

The combi backup as you call it starts heating water as soon as the water temperature in the tank falls below your preset temperature that could be just a few seconds after you start drawing water. You will never use all the water in the tank if it's designed properly. The recovery times will exceed any usage that you could have unless you filled more than 2 baths in a row without actually jumping in and bathing between bath fills (assuming 85 litres per bath). You could run a shower and never run out.

The cheapest method in my opinion is to set the tank temperature as low as you can. I think from memory it's 40 degrees. Don't worry about legionella as the Wednesday morning boil cycle to 70 degrees will kill all that. The heat losses from the tank will be very low maybe no more than just a couple of kWH per day. So it will cost about £2 per month to have 170 litres of warm water on hand ready to use.

jalexa

MidlandsMJ wrote: »

It claims to be the most efficient on the market ('SEDBUK A rated with NOx Class 5 rating - the best possible') and has a 170 litre storage tank.

Don't think so. It has small storage tank which in conjuction with the boiler capacity to reheat provides an "equivalent delivery" to a 170l indirect cylinder.

IMO an over complex concept unless merited by the intended use. Since you use "very little water daily" I would say that a traditional combi mode is adequate (if that is a user selectable feature of the boiler)

HappyMJ

jalexa wrote: »

Don't think so. It has small storage tank which in conjuction with the boiler capacity to reheat provides an "equivalent delivery" to a 170l indirect cylinder.

IMO an over complex concept unless merited by the intended use. Since you use "very little water daily" I would say that a traditional combi mode is adequate (if that is a user-feature of the boiler)

It's not over complex it's just a system boiler with a cylinder storing hot water all in one box. A true combi would not be suitable for a house with 3 bathrooms as the amount of water supplied would not be sufficient if everyone wanted a bath or shower simultaneously.

It doesn't have a traditional combi mode feature. Calling it a combi is a misdescription.

MidlandsMJ

HappyMJ wrote: »

The cheapest method in my opinion is to set the tank temperature as low as you can. I think from memory it's 40 degrees. Don't worry about legionella as the Wednesday morning boil cycle to 70 degrees will kill all that. The heat losses from the tank will be very low maybe no more than just a couple of kWH per day. So it will cost about £2 per month to have 170 litres of warm water on hand ready to use.

Very useful advice - thank you. My use of the word 'backup combi' is taken directly from the product page on the Glowworm website

jalexa wrote: »

IMO an over complex concept unless merited by the intended use. Since you use "very little water daily" I would say that a traditional combi mode is adequate (if that is a user selectable feature of the boiler)

It's a bit late in the day for an alternative boiler choice I'm afraid.